A single unit capacity of a synchronous generator has increased with an increase of the demand for electric power in recent years. As the single unit capacity of the synchronous generator becomes larger, a field current required by a generator main body becomes larger, for example, to be several thousands of amperes. For this reason, the capacity and the size of an alternating current exciter for supplying the field current to the generator main body tends to increase.
Therefore, a brushless synchronous generator including the alternating current exciter and a rotary rectifier is attracting attention. When the brushless synchronous generator is a bipolar device, an excitation capacity of the brushless synchronous generator is about 1/100 to 1/300 of that of a conventional synchronous generator. Accordingly, the brushless synchronous generator has an advantage to easily increase the capacity of the generator. Furthermore, since the brushless synchronous generator includes no brush, the brushless synchronous generator has an advantage that generation of stains due to abrasion of the brush can be avoided.